The present invention relates to communications systems, and more particularly, to volume control in communications systems including one or more linear cancelation devices.
Many bi-directional communications systems of today utilize some form of echo and/or noise suppression. For example, many handsfree telephones now include a linear echo canceler to prevent loudspeaker output from being fed back to a proximate microphone, as such feedback can be annoying or even intolerable to a far-end telephone user. U.S. patent application Ser. No. 09/005,145, filed Jan. 9, 1998 and entitled Methods and Apparatus for Providing Comfort Noise in Communications Systems, includes a detailed description of the implementation and advantages of such echo cancelation.
In addition to echo and noise suppression, loudspeaker volume control is also an important design consideration in the context of handsfree telephony. For typical applications, the loudspeaker volume control signal must compensate for a variety of factors, including component gain tolerances on both the transmitting and receiving sides of a communications link (typically in the range of +/xe2x88x9210 dB), voice power differences between human users (from xe2x88x9213 dB for a soft female to +8 dB for a loud male relative to a nominal male voice), varying loudspeaker distances across different installations (typically +/xe2x88x9212 dB for a 10-160 cm range in an automobile), and changes in background noise (typically 0-12 dB compensation for an automobile). When all such gain variations are considered, a volume control range of 77 dB is conceivable.
In conventional handsfree equipment, volume control is performed exclusively in the analog domain. In other words, a single adjustable-gain analog multiplier, or programmable gain amplifier (PGA), is used to adjust the amplitude of an analog loudspeaker feed signal. However, typical programmable gain amplifiers have nowhere near 77 dB of dynamic range. More often, programmable gain amplifiers include a dynamic range on the order of 20-25 dB.
While such range is perhaps sufficient for handheld applications (where the loudspeaker is at a fixed location relative to the user""s ear, and where the telephone body blocks out most background noise from the user""s ear), it is usually quite insufficient for handsfree applications, especially in automobiles. As a result, the telephone user experiences the product as not loud enough for a soft female voice in a noisy car with the loudspeaker at the farthest supported distance, or as too loud for a strong male voice in a quiet car with the loudspeaker at the closest supported distance, or both.
To compensate for this problem, some digital handsfree telephones of today include both analog and digital volume control. Such phones not only provide an adjustable-gain analog multiplier for the analog loudspeaker feed signal, but also include an adjustable-gain digital multiplier for adjusting the corresponding digital audio signal prior to digital-to-analog conversion. Consequently, these phones could provide an overall dynamic range more on the order of the above described 77 dB requirement.
Irrespective of whether the telephone employs analog-only volume control or dual analog and digital volume control, however, conventional volume control techniques can have a deleterious effect on telephone operation. For example, conventional techniques can lead to analog and/or digital signal clipping which can in turn significantly degrade the performance of a linear echo canceler. Moreover, analog gain changes made during a telephone call cause a linear echo canceler to retrain and therefore also result in periods of reduced echo cancelation.
Consequently, there is a need for improved techniques for providing volume control in communications devices incorporating a linear canceler.
The present invention fulfills the above-described and other needs by providing volume control techniques, wherein both analog and digital multipliers are used to adjust a loudspeaker input signal, and wherein analog volume adjustments are disabled during telephone calls. According to the invention, user adjustments to both the digital and analog gains are permitted when no call is in progress (e.g., during standby mode or at telephone installation), but only adjustments to the digital gain are permitted during a call. By providing dual (digital and analog) user controls, one of which is disabled when a call is in progress, embodiments of the invention provide the wide dynamic range required in handsfree applications while avoiding the pitfalls associated with conventional techniques. For example, volume adjustments made in accordance with the invention do not degrade linear echo canceler performance.
In an exemplary embodiment, a bi-directional communications device according to the invention includes a near-end signal processing path and a far-end signal processing path, the far-end signal processing path including a digital-domain multiplier and an analog-domain multiplier, and an echo canceler receiving a far-end digital reference signal from the far-end signal processing path and providing an echo estimate to the near-end signal processing path. According to the embodiment, both an analog gain factor of the analog-domain multiplier and a digital gain factor of the digital-domain multiplier are adjustable in dependence upon user input when users of the device are not communicating with one another via the device. However, when users of the device are communicating with one another via the device, the analog gain factor of the analog-domain multiplier is fixed while the digital gain factor of the digital-domain multiplier remains adjustable in dependence upon user input.
An exemplary method of processing signals in a telephone, the telephone including near-end and far-end signal processing paths and an echo canceler, and the far-end signal processing path including a digital domain multiplier and an analog domain multiplier, includes the steps of determining whether a telephone call is in progress, permitting adjustments of both a digital gain of the digital domain multiplier and an analog gain of the analog domain multiplier when the determination is negative, and permitting adjustments of the digital gain while blocking adjustments of the analog gain when the determination is affirmative.
The above-described and other features and advantages of the invention are explained in detail hereinafter with reference to the illustrative examples shown in the accompanying drawings. Those of skill in the art will appreciate that the described embodiments are provided for purposes of illustration and understanding and that numerous equivalent embodiments are contemplated herein.